This invention relates, generally, to a syntactic foam-core composite and, more particularly, to a syntactic foam-core material characterized by ultra-low density, broad temperature range utility, electromagnetic attenuation tailorability, low cost and convenient manufacture.
Foam-core composites are used widely in industry in an array of practical applications. For example, because of the light weight and expanded cellular structure which characterize these materials, they are used extensively as thermal, electrical and acoustical insulators and as filler materials.
Further, a trend has developed to substitute lighter and less dense materials such as aluminum and related alloys in structural applications which once required iron and steel. This trend continues in that lower density synthetic materials and composites which are desirable due to their strength and resistance to impact are now being combined with and often substituted for structural materials such as aluminum and metal alloys. In this regard, foam-core composites have found particular beneficial application in aerospace applications where low density materials are particularly beneficial.
Reticulated honeycomb and blown-foam materials have preceded foam-core composites as first and second generation synthetic materials, respectively, which have been widely used as structural materials in the aerospace industry.
First generation reticulated honeycomb materials generally have been polymeric materials manufactured in sheet form. These materials derived their low density, and thus their desirability, principally from the numerous open spaces or voids that separate the interconnected walls of the material. The reticulated honeycomb material functions well as a structural filler particularly when sandwiched and adhesively bonded between planar facing sheets or skin materials.
However, reticulated honeycomb also has shortcomings, such as its resistance to deformation. Because the material is only slightly bendable, it is unsuitable, without modification, for use in applications requiring complex shaping and acute bending as is commonly required of component parts used in aerospace applications such as intake and exhaust ducting and the like. Adapting reticulated honeycomb to such applications where complex shaping and acute bending is required has previously required that the material be "stress-scored" to make it conformable. Scoring honeycomb, however, diminishes the integrity and strength of the material and often requires that a cementitious filler be applied within the score to restore some of the structural integrity of the honeycomb. The inclusion of filler materials of this sort are not altogether desirable in that they present the possibility of delamination with the passage of time and can also impart undesirable electrical properties to the overall component in which the honeycomb is used.
The second generation expanded foam-core materials typically comprise synthetic thermoplastic resins which include polyurethane foams and expanded polystyrene. These expanded or "blown" foams achieve their open, macrocellular structure usually by incorporation of a volatile blowing agent such as pentane which causes the material to expand when sufficiently heated.
Although expanded foams have demonstrated desirable strength and insulating characteristics, such materials are generally resistant to shape conformation as is the honeycomb. Thus, expanded foams are usually best suited for applications requiring a flat, sheet material.
Another drawback relating to expanded foams relates to their tendency to expand and contract when exposed to humid and arid conditions, respectively. This sponge-like behavior is undesirable as it causes the material to increase in weight and density substantially when wet and, with repeated expansion and contraction, often results in material disintegration and delamination from facing materials to which the forms are typically adhered.
Syntactic foams are third generation materials which are believed to provide substantially improved characteristics when compared to the features associated with predecessor synthetic structural and filler materials as described above.
Generally, syntactic foams are composite materials comprised of rigid, hollow microspheres which are adhered to one another by a bonding agent. Syntactic foams differ from blown foams, such as polystyrene foam or polyurethane foam, in that the cells of syntactic foams are formed by the incorporation of small diameter, rigid microspheres into a bonding agent (typically a resin binder) rather than by expansion of the foamed material by a volatile blowing agent such as pentane. Further, voids are created between the microspheres provided that the bonding agent does not completely fill these spaces.
The bonding agents typically used in making syntactic foams are either thermosetting resins or thermoplastic resins. Examples of the thermosetting resins used as bonding agents include epoxy resins, bismaleimides, cyanates, unsaturated polyesters, non-cellular polyurethanes, thermosetting polyimides and the like. Typical examples of thermoplastic resins used as bonding agents include polyaryletherketones, polyphenylenesulfide, polyimides, polyetherimides and aromatic and aliphatic nylons.
Microspheres, which are also referred to as microbubbles, are generally rigid, hollow spheres of glass, carbon, polystyrene or phenolic resins. Among the most commonly used hollow spheres are glass microspheres which are available commercially in particle densities ranging from about 0.1 to about 0.6 gm/cc and diameters within the range of about 5 to about 200 microns.
Methods are known in the art for making "loaded" syntactic foam-core materials which have increased radar absorption capability. The loaded materials are formed of dielectric materials which are presented in a characteristic web-like structure that enhance the overall radar absorption of the material.
Various disclosures of prior art syntactic foam-core materials and the methods for making these materials are provided in the following U.S. Pat. Nos.: 4,410,639 to Bouley, et al.; 4,412,012 to Bouley, et al.; 4,482,590 to Bouley, et al.; 4,548,861 to Barnes, et al.; 4,568,603 to Oldham; 4,595,623 to Du Pont, et al.; 4,861,649 to Browne; 4,956,393 to Boyd, et al.; 5,120,769 to Dyksterhouse, et al.; 5,135,959 to Hill; 5,167,870 to Boyd, et al.; 5,185,381 to Ruffoni; 5,252,632 to Savin; and 5,356,958 to Matthews. All of the foregoing patents are herein incorporated by reference.
The related references in the art indicate that considerable effort has been directed toward formulating strong and low density syntactic foam-core materials which are capable of being loaded with electromagnetic attenuating fillers. None of the above-identified disclosures, however, reveals a highly shape-mutable syntactic foam-core material that is characterized by high strength, ultra-low density, broad temperature range utility and electromagnetic attenuation tailorability and which is relatively simple and inexpensive to produce and does not require sophisticated manufacturing equipment. Thus, the related art notwithstanding, a need which is unsatisfied by known technology exists within the art for the present invention which accomplishes these and other advantageous objectives.